As is known, polyamides generally exhibit a balance of thermal, strength and stiffness properties which make them suitable for many applications. However, impact strengths typically are low as measured by mechanical tests such as the Notched Izod Impact Test or the high speed instrumented Dart Impact Test. In some cases, elongation measured in tensile testing is also relatively low. Improvements in impact strength and elongation are needed for better performance of articles fabricated from polyamides for use in high impact applications and especially those in which the articles may be subject to cracks or have imperfections. Polyamides of improved impact strength over a broad range of temperatures would be particularly desirable.
Crystalline polyamides comprising at least about 50 mole percent aliphatic terephthalamide units exhibit particularly good thermal properties making them useful in relatively high temperature applications such as under-the-hood automotive parts. Other desirable properties of such compositions include their high strength and stiffness. However, impact strengths are relatively low. Further, such polyamides have relatively high melting points, e.g., about 290.degree. C. or higher, and degradation temperatures of some of the materials do not greatly exceed their melting points; accordingly, requirements for melt processing the compositions are more rigorous and complex than those for lower melting polyamides such as nylon 66 which melts at about 260-265.degree. C.
Various additives have been proposed for improving impact strength of polyamides. Generally, these additives are rubbery compositions that can be blended or incorporated into the polyamides to provide improvements in impact strength. Of course, it also is important that such improvements be achieved without substantial adverse effects on desirable properties of the polyamide. It also is important that the additives retain their effectiveness after processing of polyamide compositions containing the same and during use of articles prepared therefrom.
U.S. Pat. No. 4,427,828 issued Jan. 24, 1984, and U.S. Pat. No. 4,508,874, issued Apr. 2, 1985, both to Hergenrother et al., disclose impact resistant polyamide compositions containing about 50-90 wt. % polyamide having a number average molecular weight of at least 10,000 and about 10-50 wt. % of a pendant succinic anhydride group-containing reaction product of maleic anhydride with a hydrogenated polymer of a conjugated diene or hydrogenated random or block copolymer of a conjugated diene and a vinyl aromatic hydrocarbon. The disclosed hydrogenated polymer or copolymer has about 0.5-20% of its original unsaturation content before hydrogenation and at least 5 wt. % of the polyamide and the maleic anhydride-hydrogenated polymer or copolymer reaction product is present in the form of a graft polymer containing at least 20% polyamide. Polyamides mentioned in the patent include those prepared from various aminocarboxylic acids or lactams thereof or from dicarboxylic acids and diamines, including polycaprolactam, poly(hexamethylene adipamide), poly(hexamethylene isophthalamide), "poly(methoxylylene adipamide)" and hexamethylene adipamide/caprolactam, hexamethylene adipamide/hexamethylene isophthalamide, hexamethylene adipamide/hexamethylene terephthalamide, hexamethylene adipamide/hexamethylene azeleamide copolyamides and hexamethylene adipamide/hexamethylene azeleamide/caprolactam terpolyamides. Examples of various maleic anhydride-hydrogenated diene polymer and diene-vinyl aromatic copolymer adducts also are given. Included are those in which the diene is butadiene and the vinyl aromatic is styrene.
U.S. Pat. No. 4,783,503 issued Nov. 8, 1988, to Gergen et al. discloses impact resistant blends of a polyamide and a thermally stable, modified, selectively hydrogenated, block copolymer of a conjugated diene and a vinyl aromatic compound. The polyamides are semi-crystalline or amorphous resins having molecular weights of at least 5000. Polyamides named in the patent include poly(hexamethylene adipamide), poly(hexamethylene dodecaneamide), polycaprolactam, and adipic acid/isophthalic acid/hexamethylene diamine copolyamides. The disclosed, modified, selectively hydrogenated, block copolymer component has residual unsaturation in the polydiene block of about 0.5-20% of its original unsaturation content and is grafted in the vinylarene block with an electrophilic group. Examples of various dienes, vinyl aromatics and electrophiles are disclosed and the examples of the patent illustrate an acid-functionalized styrene-ethylene/butylene-styrene block copolymer containing 29 wt. % styrene and blends thereof with a commercial nylon 66 prepared at temperatures up to 285.degree. C. The modified, hydrogenated block copolymers are said to exhibit greater thermal stability than materials prepared by the ENE process such as in Hergenrother et al.
Published European Patent Application 86201336.4 (Publication No. 0 211 467) of Gelles et al., published Feb. 25, 1987, discloses impact resistant polymeric compositions containing a polyamide or polyester and a thermally stable, acid-functionalized, selectively hydrogenated block copolymer of a vinyl aromatic compound and a conjugated diene in which unsaturation in the polydiene block is less than 10% of its original unsaturation and substantially all of the acid groups or derivatives thereof are grafted to a secondary or tertiary carbon of the polydiene group. Various polyamides, including poly(hexamethylene adipamide), poly(hexamethylene isophthalamide), poly(hexamethylene dodecaneamide), polycaprolactam and adipic acid/isophthalic acid/hexamethylene diamine and polyhexamethylene ter-co-isophthalamide copolyamides, are mentioned. Various diene and vinyl aromatic monomers are disclosed for preparation of block copolymers, 1,3-butadiene and styrene, respectively, being identified as preferred. The polydiene block is selectively hydrogenated and the hydrogenated result is grafted with an acid group or derivative to provide functionality. Preferred monomers for the grafting reaction are said to be maleic anhydride, maleic acid, fumeric acid and their derivatives and sulfonic acids. The examples illustrate maleic anhydride-functionalized styrene-ethylene/butylene-styrene block copolymer blends with nylon 66 prepared at temperatures up to 285.degree. C. and having improved Izod impact strength relative to the neat nylon 66. Like the modified, hydrogenated block copolymers used according to Gergen et al., those of Gelles et al. are said to exhibit improved thermal stability relative to modified polymers prepared by the ENE process such as those according to Hergenrother et al.
A published advertisement for Kraton.RTM. FG 1901X by Shell Chemical Company, appearing in Plastics Compounding March/April, 1989, discloses a block copolymer having styrene and ethylene/butylene polymer blocks functionalized with maleic anhydride and utility thereof as an impact modifier for engineering thermoplastics, as a tie layer and as a compatibilizer for dissimilar scrap thermoplastics. Improvement in Izod impact strength resulting from incorporation of the material into nylon 66 also is reported in the advertisement as is the material's ability to withstand processing temperatures of engineering thermoplastics.
British Patent Specification 998,439 to E.I. dupont de Nemours and Company discloses blends of polyamides with olefin copolymers containing acid groups. The blends are described as having improved toughness as measured by the Izod Impact Test. The polyamides used according to this specification are derived from dicarboxylic acids and diamines, with nylon 66, 610, 6, 66/610 and 66/6 identified as preferred materials. The olefin copolymers are derived from 1-olefins, such as ethylene, propylene, butene-1, isobutene and styrenes or mixtures of 1-olefins, and an acidic component which may be incorporated by copolymerization with the olefin or by grafting to a polyolefin. Acidic components include alpha, beta-unsaturated carboxylic acids, such as acrylic acid, methacrylic acid, ethacrylic acid, maleic acid, and esters and anhydrides thereof and sulfonic acid group-providing compounds such as concentrated sulfuric acid and SO.sub.2 Cl.sub.2. The olefin-acid group copolymer and polyamides are blended to finely disperse the former in the polyamide. The blends are described as incompatible but showing evidence of reaction between amine or acid end groups of the polyamide and acid groups of the olefin-acid component. The blends are said to contain 1-50% olefin-acid polymer and 50-90% polyamide.
U.S. Pat. No. 3,780,140 to Hammer, issued Dec. 18, 1973, discloses blends of ethylene-carbon monoxide-termonomer polymers with organic polymers, including polyamides, to improve properties of the latter in various respects, including toughness. As. termonomers used in preparing the polymers, unsaturated mono- and dicarboxylic acids of 3-20 carbon atoms and esters thereof, vinyl esters of C.sub.1-18 saturated acids, vinyl C.sub.1-18 alkyl ethers, acrylonitrile, methacrylonitrile and copolymerizable unsaturated hydrocarbons such as C.sub.3-12 alpha-olefins are disclosed.
U.S. Pat. No. 4,174,358 to Epstein, issued Nov. 13, 1979, discloses compositions containing a polyamide having a molecular weight of at least 5,000 prepared from saturated C.sub.4-12 dicarboxylic acids and C.sub.4-15 diamines and a polymer dispersed in the polyamide as discrete particles 0.01 to 1 micron in size, having sites that adhere to the polyamide and tensile modulus of 1 to 20,000 and wherein the ratio of the polyamide tensile modulus to that of the polymer is greater than 10:1. The polymers dispersed in the polyamide are said to be represented by the formula A.sub.(a) --B.sub.(b) --C.sub.(c) --D.sub.(d) --E.sub.(e) --F.sub.(f) --G.sub.(g) --H.sub.(h) wherein (a)-(h) are specified numerical values and repeat units A-H are also specified monomers from which the polymers are said to be derived. Compositions according to this patent are said to have improved ductility as measured by the Izod Impact Test.
U.S. Pat. No. 3,842,029 to Yokohama et al., issued Oct. 15, 1974, discloses noise, skid and impact resistant compositions containing glass fibers, certain thermoplastic polymers, including polyamides, and conjugated diene-monovinyl aromatic block copolymers. Disclosed polyamides include nylon 6, 66, 7, 610, 612, 11 and 12. Disclosed block copolymers include styrene-butadiene, styrene-isoprene, 1,3-pentadiene-vinyl toluene or -vinyl naphthalene copolymers.
U.S. Pat. No. 4,423,186 to Grigo et al., issued Dec. 27, 1983, discloses impact resistant polyamide molding compositions containing 35-99 weight % polyamide and 1-65 weight % of a partly crosslinked mixture of 5-80 weight % ethylene/(meth)acrylic acid (esters) and 95-20 weight % polybutadiene rubber. Disclosed polyamides are nylon 6, 66 and 6/66 copolymers.
Although these patents and publications disclose various additives for improving impact strength and ductility of various polyamides, there remains a need for improvement in such properties of polyamides containing a substantial proportion of terephthalamide units, particularly in view of the relatively high melting points and often narrow melt processing temperature ranges thereof which limit utility and effectiveness of many materials heretofore proposed as impact modifiers. In this regard, it is noted that Gergen et al. and Gelles, despite emphasizing improved thermal stability of their additives, as discussed hereinabove, illustrate use of such additives only with lower melting nylon 66 resins at temperatures of 285.degree. C. or lower in their respective examples. Similarly, while the Shell Chemical Company advertisement for Kraton.RTM. FG 1901X mentions capability of the material to stand up to high processing temperatures, the supplier's Material Safety Data Sheet (MSDS 2,898-1, Jun. 22, 1987) for the product cautions against processing above 287.degree. C.
An object of this invention is to provide modified polyamide compositions. Another object is to provide toughened polyphthalamide compositions. Another object is to provide polyphthalamide compositions having a desirable balance of properties including impact strength, tensile strength and thermal properties. Other objects are to provide a method for preparing such compositions and articles fabricated therefrom. Other objects will appear to those skilled in the art from the description and claims set forth herein.
The objects of this invention can be attained by providing polyamide compositions comprising a crystallizable polyamide component comprising at least about 50 mole percent recurring aliphatic terephthalamide units and a modifying component comprising a block copolymer having pendant succinic anhydride groups wherein the block copolymer comprises polymerized styrene blocks and rubbery blocks comprising ethylene/propylene, ethylene/butylene, or ethylene/pentylene polymer blocks or combinations thereof. In another aspect, these objects are attained by providing improved compositions comprising such a polyphthalamide component and a modifying component comprising the functionalized block copolymer as described above and, further, at least one additional rubbery polymer. Advantageously, the invented compositions exhibit significant improvements in toughness, including impact strength as determined by the Izod impact test and ductility as indicated by elongation during tensile testing, relative to the base polyphthalamide compositions. These improvements are frequently achieved despite preparation and processing of the invented compositions at temperatures approaching or even exceeding nominal degradation temperatures of the modifier components. Further, improvements in toughness can be achieved at relatively low levels of the functionalized, block copolymer component and without substantial loss of thermal properties such as heat deflection temperature. In addition the compositions not only retain their desirable properties, including improved Izod impact strengths, over a wide temperature range up to at least about 120.degree. C., but also exhibit excellent retention of tensile strength, flexural modulus, and Izod impact strength when wet or subjected to humid environments.